In the recovery of hydrocarbons, such as oil and gas, from natural hydrocarbon reservoirs, extensive use is made of wellbore treatment fluids such as drilling fluids, completion fluids, work over fluids, packer fluids, fracturing fluids, conformance or permeability control fluids and the like.
In many cases significant components of wellbore fluids are thickening agents, usually based on polymers or viscoelastic surfactants, which serve to control the viscosity of the fluids. Typical viscoelastic surfactants are N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride and potassium oleate, solutions of which form gels when mixed with corresponding activators such as sodium salicylate and potassium chloride.
Conventional surfactant molecules are characterized by having one long hydrocarbon chain per surfactant headgroup. In the viscoelastic gelled state these molecules aggregate into worm-like micelles. Gel breakdown occurs rapidly when the fluid contacts hydrocarbons which cause the micelles to change structure or disband.
In practical terms the surfactants act as reversible thickening agents so that, on placement in subterranean reservoir formations, the viscosity of a wellbore fluid containing such a surfactant varies significantly between water- or hydrocarbon-bearing zones of the formations. In this way the fluid is able preferentially to penetrate hydrocarbon-bearing zones.
The application of viscoelastic surfactants in both non-foamed and foamed fluids used for fracturing subterranean formations has been described in several patents, e.g. EP-A-0835983, U.S. Pat. No. 5,258,137, U.S. Pat. No. 5,551,516, U.S. Pat. No. 5,964,295 and U.S. Pat. No. 5,979,557.
The use of viscoelastic surfactants for water shut off treatments and for selective acidizing is discussed in GB-A-2332224 and Chang F. F., Love T., Affeld C. J., Blevins J. B., Thomas R. L. and Fu D. K., “Case study of a novel acid diversion technique in carbonate reservoirs”, Society of Petroleum Engineers, 56529, (1999).
A problem associated with the use of viscoelastic surfactants is that stable oil-in-water emulsions can be formed between the low viscosity surfactant solution (i.e. broken gel) and the reservoir hydrocarbons. As a consequence, a clean separation of the two phases can be difficult to achieve, complicating clean up of wellbore fluids. A factor promoting emulsion formation is believed to be a reduction of the oil/water interfacial energy caused by a tendency for the surfactant molecules to collect at the water/oil interface.
The recovery of hydrocarbons, such as oil and gas, from a subterranean well formation can be impeded by scales obstructing the flow of hydrocarbons from hydrocarbon-bearing zones of the formation. Typical scales are barite (BaSO4) or calcite (CaCO3) and it is common practice to treat these by bull-heading an aqueous-based scale dissolver fluid through a well bore and into the formation.
For example, one conventional scale dissolver for barite scale consists of a concentrated solution of potassium carbonate, potassium hydroxide and the potassium salt of ethylenediaminetetraacetic acid (EDTA), the corrosive and chelating nature of the solution being effective in removing scale. Carbonate scales may be dissolved using simple mineral acids, such as HCl.
However, hydrocarbon-producing wells often contain zones that are watered-out, producing only, or very largely, water. If the scale dissolver enters these zones, scale may also be removed therefrom. This can lead to an undesirable increase in the water cut of the fluid produced by the well.
Dimer surfactants have found some application in fluids used in the exploration and production of hydrocarbons. B. A. M. Oude Alink, “Fatty acids in oil field chemicals” in Fatty Acids in Industry, eds. R. W. Johnson and E. Fritz, pp. 407-429, Marcel Dekker, New York, (1989) and Henkel Corporation Chemicals Group, Abstracts of Dimer Acid Use—Patents and Journal References, Vol. 1, Technical Bulletin 109A, 1968 review the use of dimer oleic acids in the production of corrosion inhibitors, lubricants for water-based drilling fluids and emulsifying surfactants for invert emulsion oil-based drilling fluids. U.S. Pat. No. 4,108,779 describes the use of (apparently calcium salts of) oleic acid dimers to control the viscosity of water-in-oil spacer fluids. U.S. Pat. No. 4,607,700 and U.S. Pat. No. 5,193,618 describe the use of a dimer of an alphaolefin sulphonate surfactant to form a foam steam drive injection fluid for hydrocarbon discovery.